The present invention relates to a color cathode ray tube and, more particularly, to a color cathode ray tube having an electron gun in which a cathode structure arrayed in line within a cup-shaped first grid electrode is fixedly housed.
Color cathode ray tubes having a plurality of cathodes arrayed in line are generally used as image display devices for television receivers or monitors of data processing terminals.
This kind of cathode ray tube (CRT) has an evacuated envelope comprising a panel portion having a phosphor screen formed on its inner surface, a neck portion which houses an electron gun structure, and a funnel portion which connects the panel portion and the neck portion. A widely used type of electron gun structure is an inline type electron gun structure constructed to emit three electron beams toward the phosphor screen in a horizontal plane.
FIG. 7 is a view illustrating an example of a typical electrode gun for use in a cathode ray tube, which electron gun has a construction in which a cathode support and a first grid electrode are fixed. In FIG. 7, the electron gun has a cathode support 15 provided with a cathode inside, a cup-shaped first grid electrode 16, welding spots 17 at which the first grid electrode 16 and the cathode support 15 are welded to each other, electron beam passing holes 18 provided in the first grid electrode 16 (reference numerals denote 18S side electron beam passing holes, reference numeral 18C denotes a center electron beam passing hole), and a bead portion 19 to be buried into a bead glass to fix the first grid electrode 16.
The cathode support 15 is inserted inside of the cup-shaped first grid electrode 16 and is welded at the welding spots 17 in its open end portion. The welding spots 17 are also present in a back portion which is not shown in FIG. 7, so that first grid electrode 16 and the cathode support 15 are fixed to each other at four spots.
FIG. 8 is a cross-sectional view, taken in an inline direction, of a triode portion of the electron gun. Symbols K denote cathodes, and reference numerals 20 denote cathode structures each provided with a cathode K for emitting an electron beam toward the first grid electrode 16 (reference numeral 20S denotes side cathode structures, reference numeral 20C denotes a center cathode structure). The structure includes sleeves 21 to which the respective cathode structures are fixed, and a hermetic glass (insulating substrate) 22. The first grid electrode 16 is a cup-shaped electrode in which the cathode support 15 is housed. Each of the cathode structures 20 is fixed in an electrically insulated state by the hermetic glass 22, and is fixed to the cathode support 15.
During the start-up period of the cathode ray tube, each of the cathode structures 20 is heated by a heater which is not shown. Each of the cathode structures 20 is thermally expanded by this heating and the distance between the cathodes K and the electron beam passing holes of the first grid electrode 16 becomes smaller, so that a larger amount of cathode current flows. Then, the first grid electrode 16 is thermally expanded and the distance between the cathodes K and the electron beam passing holes of the first grid electrode 16 becomes longer, so that the cathode current becomes gradually less. After that, the thermal expansion of the cathode structures 20 and that of the first grid electrode 16 comes to an end and the distance between the cathodes K and the first grid electrode 16 stabilizes at a constant value, so that the brightness on the screen becomes constant.
The first grid electrode 16 and the cathode support 15 used in the illustrated electron gun differ from each other in coefficient of linear thermal expansion (hereinafter referred to as the coefficient of thermal expansion). During the operation of the CRT, the electron gun is heated at a high temperature. In the electron gun constructed in this manner, since the first grid electrode 16 and the cathode support 15 are fixedly welded to each other, the first grid electrode 16 and the cathode support 15 are deformed by the difference between their coefficients of thermal expansion.
In the electron gun structure which constitutes the above-described electron gun, the amounts of thermal expansion assume the relationship of the first grid electrode 16 greater than the cathode support 15. In this case, when the first grid electrode 16 is expanded, the cup-shaped first grid electrode 16 pulls the cathode support 15 in the directions indicated by arrows in FIG. 7. Since the first grid electrode 16 is fixed to the bead glass by the bead portion 19, the cathode support is deformed in the direction in which the central portion of the cathode support 15 approaches the first grid electrode 16 compared to the edge portion of the same. Accordingly, the cathode surfaces of the cathode structures 20 fixed to the cathode support 15 approach the first grid electrode 16. Specifically, the distance between the cathode surface of the center cathode structure 20C and the first grid electrode 16 becomes shorter than the distance between the cathode surface of the side cathode structures 20S and the first grid electrode 16.
FIG. 9 is a graph which shows a variation in cathode current with time, wherein the vertical axis represents cathode current and the horizontal axis represents time.
Reference numeral 23 denotes a variation in the cathode current of the center cathode, and reference numeral 24 denotes a variation in the cathode current of each side cathode.
For example, FIG. 9 shows variations in cathode currents with time in a cathode ray tube of xcfx8629 neck in which the cathode support 15 is made of 42% Nixe2x80x94Fe (coefficient of thermal expansion: 46xc3x9710xe2x88x927/xc2x0 C.) and the first grid electrode 16 is made of 50% NIxe2x80x94Fe (coefficient of thermal expansion: 100xc3x9710xe2x88x927/xc2x0 C.). As shown in FIG. 9, when about 10 minutes passes after power is turned on, the gap between the cathode surfaces and the first grid electrode becomes stable and the cathode currents become constant. For this reason, the best cathode current value (set value) is set to the value of each of the cathode currents obtained when about 10 minutes passes after power is turned on. As shown in FIG. 9, according to the welding positions in the above-described structure, when about 1 minute passes after power is turned on, the cathode current at the center cathode reaches 115% of the set value and the cathode current at each of the side cathodes reaches 150% of the set value, and the difference in cathode current between the center cathode and each of the side cathodes is about 35%. Normally, in the electron gun of xcfx8629, the difference in cathode current between the center and side cathodes reaches its maximum in about 1 minute after power is turned on, but in the case of the welding positions in the above-described structure, the difference in cathode current between the center and side cathodes reaches a maximum of 35% until the cathode currents become stable after power is turned on. The related art cathode ray tube has the problem that at the starting time of its operation, the difference between the cathode current of the center cathode and the cathode current of each of the side cathodes is so large that no desired colors can be displayed on the screen. In other words, in the related art cathode ray tube, the manner of variation of the distance between the cathode surface at the center portion and the first grid electrode differs from the manner of variation of the distance between the cathode surface at each side portion and the first grid electrode, so that it is difficult to stably supply electron beams to the phosphor screen.
It has recently been proposed to provide a cathode ray tube in which the sensitivity of its deflection yokes to electron beams is increased to reduce the power consumed for deflecting the electron beams. Such a cathode ray tube has a reduced neck diameter. However, the electron gun of the cathode ray tube has the disadvantage that a cutoff voltage which is determined by the distance between the cathodes and the first grid electrode becomes so sensitive that adjustment of the cutoff voltage becomes difficult.
The invention provides a color cathode ray tube provided with an electron gun which is capable of making more uniform a variation in the gap between the cathodes and the first grid electrode at the center portion and at each side portion, and making more uniform the amounts of cathode currents of the center cathode and each side cathode during the start-up period of the cathode ray tube, thereby maintaining color balance on the screen. The invention also provides a color cathode ray tube provided with an electron gun which is capable of restraining a variation in the distance between the cathodes and the first grid electrode and reducing a variation in brightness during a long-time operation of the cathode ray tube.
To make more uniform the amount of variation with time in the gap between the cathodes and the first grid electrode at the center portion and at each side portion, it is necessary to make more uniform the thermal deformation of a cathode support in the inline direction thereof.
For this purpose, the invention provides a color cathode ray tube which includes: an evacuated envelope including a panel portion on which a phosphor screen is formed, a neck portion, and a funnel portion which connects the panel portion and the neck portion; and an electron gun having at least an electron beam generating unit which generates three electron beams toward the phosphor screen in a horizontal plane, the electron beam generating unit being housed in the neck portion and being made of cathodes, a first grid electrode and an accelerating electrode, the electron gun further including a plurality of electrodes fixedly buried in an insulating material in a predetermined array and at predetermined intervals in a tube axis direction. The first grid electrode has a cup-like shape, and each cathode structure is fixed to a cathode support in an electrically insulated state by glass. Each of the cathode support and the first grid electrode has a rectangular or elliptical face, and the first grid electrode houses the cathode support, and a fixing portion for fixing the first grid electrode and the cathode support to each other is located in a shorter-side portion.
Otherwise, the fixing portion for fixing the first grid electrode and the cathode support to each other is welded on an axis along which the cathode structures are arrayed (hereinafter referred to as the inline axis).
According to the above-described construction, the amount of variation in the distance between the cathode surface of the center portion and the first grid electrode can be made approximately equal to the amount of variation in the distance between the cathode surface of each side portion and the first grid electrode. In addition, when the cathodes are heated by heaters and electron beams are radiated from electron radiating substances lying over the electron emitting surfaces of the cathodes, it is possible to restrain thermal deformation of the cathode support in the inline direction, thereof, whereby it is possible to maintain the concentration of electron beams on the screen.